Latex paint coatings are typically applied to substrates and dried to form continuous films for decorative purposes as well as to protect the substrate. Such paint coatings are often applied to architectural interior or exterior surfaces, where the coatings are sufficiently fluid to flow out, form a continuous paint film, and dry at ambient temperatures.
A latex paint ordinarily comprises an organic polymeric binder, i.e., latex, pigments, and various paint additives. In dried paint films, the polymeric binder functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness, although some paints contain little or no opacifying pigments and are described as clear coatings. The manufacture of paints involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in a dispersant medium, and thinning to commercial standards.
Two types of copolymers commonly used in formulating latex paints include: (i) an all acrylic system, e.g., copolymers of methyl methacrylate, butyl acrylate or 2-ethylhexyl acrylate with small amounts of functional monomers, such as, carboxylic acids; and (ii) vinyl acetate-based copolymers usually in combination with a small proportion of the above-mentioned lower alkyl acrylates, such as, for example, butyl acrylates. Because of its low cost, vinyl acetate is an attractive alternative to certain acrylate monomers, e.g., methyl methacrylate, for use in architectural coating latexes. Unfortunately, vinyl acetate suffers from poor hydrolytic stability especially under alkaline conditions and accordingly, finds only limited application in exterior coatings. Alkali resistance is extremely important, for example, when paints are applied over an alkaline construction material, such as, for example, cement.
Ethylene is a desirable comonomer for polymerization with vinyl acetate to form latexes because ethylene has properties which can compensate for the shortcomings of vinyl acetate. More specifically, because of ethylene's low molecular weight, it permits a high level of introduction of non-hydrolyzable segments on a per weight basis which can improve the hydrolytic stability properties of the vinyl acetate copolymer. Stated another way, the hydrocarbon segments provided by the ethylene tend to reduce water solubility, thus imparting greater hydrolytic stability. Moreover, ethylene has a low glass transition temperature which can provide enhanced copolymer hydrophobicity and enhanced water and alkali resistance.
However, ethylene is a gas at normal temperatures and pressures and does not readily react with vinyl acetate unless at elevated pressures. The reaction normally requires special reactors with suitable wall thicknesses, pressure resistant seals and valves and other apparatus which are not necessary in conventional emulsion polymerization. Therefore, plant costs are often significantly higher for polymerizations involving ethylene than for conventional latex polymerization processes. As a result, the benefits expected from the use of ethylene can be offset or nullified by the high costs associated with using ethylene.
Accordingly, new latex copolymer compositions based on the use of alkenes, e.g., ethylene, as comonomers are desired which do not require high pressure apparatus commonly associated with the polymerization of alkene copolymers. Desirably, such compositions would have utility in a variety of applications, particularly for use as latex paints. Also, efficient processes for the production of such latex copolymers are desired which can provide enhanced levels of alkene incorporation at a given reaction pressure.